Apparatus for dry replenishment of electroless plating solutions

ABSTRACT

This invention is for an apparatus useful for replenishment of an aqueous electroless or electrolytic plating solution. The apparatus permits addition of replenishers to an electroless plating solution in essentially dry form in a manner that avoids spontaneous decomposition of the solution. The apparatus comprises an inlet means for the plating solution, means for imparting a swirling motion to the solution as it passes through the apparatus in contact with dry replenisher constituents, a porous filter container for said dry replenisher constituents and outlet means for said solution downstream from said porous filter container. By use of the apparatus of this invention, essentially dry materials are used for replenishment of a plating solution rather than liquid replenishers as in the prior art thereby avoiding volume growth of the plating solution.

[ 1 Mar. 11, 1975 1 APPARATUS FOR DRY REPLENISHMENT OF ELECTROLESS PLATING SOLUTIONS [75] Inventors: Oleh Borys Dutkewych, Medfield,

Mass.; Lebert Arthur l-lotmann, Westminster, Calif.

[73] Assignee: Shipley Company, Inc., Newton,

Mass.

[22] Filed: Dec. 7, 1972 [21] Appl. No.: 313,178

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 188,243, Oct. 12,

1971, Pat. No. 3,770,464.

[52] US. Cl 137/268, 23/272, 118/429 [51] Int. Cl. C23c 3/02 [58] Field of Search 137/268, 571, 572, 575;

1,500,096 7/1924 Oxley 23/272 X 1,815,711 7/1931 Hensel 23/272 2,700,651 l/l955 Tepas, Jr. et a1. 23/272 2,997,373 8/1961 Stephens 23/272.6 R 3,145,087 8/1964 Walker 23/272 X 3,632,415 l/1972 Franklin 210/509 X Primary Examiner-Martin P. Schwadron Assistant ExaminerRobert J. Miller Attorney, Agent, or FirmDike, Bronstein, Roberts, Cushman & Pfund 4 [57] ABSTRACT This invention is for an apparatus useful for replenishment of an aqueous electroless or electrolytic plating solution. The apparatus permits addition of rep1enish ers to an electroless plating solution in essentially dry form in a manner that avoids spontaneous decomposition of the solution. The apparatus comprises an inlet means for the plating solution, means for imparting a swirling motion to the solution as it passes through the apparatus in contact with dry replenisher constituents, a porous filter container for said dry replenisher constituents and outlet means for said solution downstream from said porous filter container. By use of the apparatus of this invention, essentially dry materials are used for replenishment of a plating solution rather than liquid replenishers as in the prior art thereby avoiding volume growth of the plating solution.

15 Claims, 2 Drawing Figures APPARATUS FOR DRY REPLENISHMENT OF ELECTROLESS PLATING SOLUTIONS Cross-Reference to Related Application This application is a continuation-in-part of copending U.S. Pat. application Ser. No. 188,243 filed Oct. 12,

BACKGROUND OF THE INVENTION 1. Introduction This invention relates to an apparatus for replenishment of an electroless plating solution.

2. Description of the Prior Art As is known in the art, electroless metal deposition refers to the chemical deposition on a receptive surface of an adherent copper coating in the absence of an external electric source. Such-deposition is useful, for example, in the manufacture of printed electric'circuits, as linings for wave-guide cavities, as an initial conductive coating in electro-forming, and for decoration.

A number of electroless plating processes and solutions have heretofore been known and are disclosed in numerous U.S. Patents including U.S. Pat. Nos. 2,874,072; 2,938,805; 3,075,856; 3,095,309; 3,222,195; and 3,392,035, all incorporated herein by reference.

Using electroless copper deposition solutions for purposes of illustrating only, it being understood that the apparatus of the invention is useful for other electroless plating solutions such as electroless nickel solutions. Such copper plating solutions typically comprise an aqueous solution of cupric ions, a source of formaldehyde, hydroxide and a chelating agent to render the cupric ions soluble in an alkaline solution. Deposition occurs by the reduction of the cupric ion to copper using formaldehyde initiated by the presence of a suitable catalytic surface, for example, various surfaces of catalyzed plastic as disclosed in U.S. Pat, No. 3,011,920.

lt is known in the art that in use of electroless or electrolytic deposition solutions, the solutions become depleted in solution constituents. For example, with regard to electroless copper solutions, cupric ion concentration becomes depleted through deposition on a substrate. Formaldehyde becomes depleted as it is used to reduce the cupric ion to metallic copper. The concentration of the chelating agent is somewhat lowered by drag-out on the parts passing out of the plating bath. Consequently, with use, as the solution becomes depleted, deposition rate decreases to a point where the solution is no longer useable.

in order to increase the useful life of an electroless plating solution, it has been the practice in the art to replenish the solution with solution constituents at frequent intervals during the plating cycle. This has been accomplished by adding one or more of the necessary constituents to the plating bath in the form of aqueous solutions.

A major difficulty with the prior art procedure for replenishment of a metal depositing solution is that there is an increase in the original solution volume each time it is replenished with a liquid replenisher. This is due to the fact that as the solution becomes depleted of its solution constitutents, there is no corresponding decrease in volume, as the only losses in volume occur as a result of evaporation and drag-out. For example, in one particular large volume continuous plating operation, there are about five replenishment cycles per 24 hour period where a replenishment cycle is defined as percent replacement of the original metal content. The solution is replenished withaqueous solution constituents to maintain the metal content at a specific level, in order to equalize the deposition rate. Each replenishment cycle results in about a 5.74 percent increase in volume of the plating solution which means thatthere is about a 28.7 percent volume increase every 24 hours. As a result, the cost of the plating process is substantially increased and a greater volume of solution must ultimately be disposed of. With code regulations preventing dumping of industrial wastes containing metal values, especially copper, this becomes an everincreasing problem.

It is known that attempts have been made in the prior art to add replenishers in dry form to avoid volume growth of theplating solution. However, these attempts have been unsuccessful because the most obvious method of adding dry ingredients to the plating solution would be to simply drop the dry powder onto the surface of the solution in a plating tank. However, this method is not operative because the powders on the solution surface act as localized areas of high concentration and as nucleating sites for plating resulting in spontaneous decomposition of the bath and subsequent loss ofthe entire plating bath. Also, particles in the plating solution hitting the work piece being plated result in defects in the coherency of the metal plate.

Statement of the Invention The subject invention provides an apparatus for replenishing a stable plating bath with replenishers that are essentially in dry form. Thus, even though the replenisher is in dry form, it can be added to the bath without causing spontaneous decomposition of the bath and without affecting the coherency of the metal plate. Moreover, the ability to dry replenish avoids the problems of extreme volume growth of solution. The invention is based upon the discovery that the replenishers may be added to a stable plating bath if they are kept out of contact with air and are rapidly dispersed and dissolved in solution. This is accomplished with the apparatus of the invention. This apparatus comprises the combination of a plating tank, with accessory equipment and a replenishment chamber external to the plating tank. In said combination, a portion of the plating solution is removed from the plating tank, passed into the replenishment chamber where replenisher is taken into solution out of contact with air and then the solution is returned to the plating tank. The replenishing chamber comprises inlet means for the plating solution which imparts a swirling motion to said solution as it passes into the replenishment chamber, a porous filter container for said dry replenisher constituents, outlet means for said solution downstream from said porous filter container and means to convey the plating solution, enriched in replenisher constituents, from the replenishment chamber back to the plating tank.

Typical replenishers that are added using the apparatus of the subject invention include one or more of a source of the plating metal ions, a solid source of the reducing agent, a chelating agent for the plating ions in solid form, pH adjustors and known additives such as stabilizers, brighteners, surfactants and the like, all of which may be in solid form. These ingredients may be added singularly or admixed with each other except that in copper plating solutions, caustic as a pH adjustor cannot be admixed with formaldehyde unless special precautions are taken as will be discussed in greater detail below. Preferably at least a salt of the plating metal in combination with the chelating agent and most preferably the stabilizers are added together so that the chelating agent may facilitate dissolution of the metal salt in the plating solution.

The use of essentially dry materials for replenishment rather than aqueous solutions as in the prior art prevents volume growth of the plating solution in use. Consequently, with the avoidance of volume growth, there is less of a problem of dumping wastes and accordingly, this invention is a contribution to pollution control efforts.

DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 represents a preferred plating apparatus in combination with the apparatus for adding dry replenisher; and I FIG. 2 represents a cross-sectional view of the apparatus of FIG. 1 taken along section line 22.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For brevity, the remainder of the description will be confined to the use of the apparatus of the invention used in conjunction with electroless copper plating solutions, it being understood that the apparatus is useful for any plating solution including electrolytic and electroless nickel and cobalt plating solutions, for example.

The term essentially dry form, as used to describe the dry replenisher herein, is intended to mean a composition having a solids content of at least 75 percent by weight. Thus, the term contemplates highly concentrated solutions, substantially more concentrated than prior art replenishers, as well as dry powders. In some instances, limited moisture content is desirable as it facilitates molding the dry replenisher to tablet or other desired shape. In this respect, it should be noted that those substances added in smaller concentrations such as stabilizers, surfactants, brighteners, chelating agents and the like may be in liquid form admixed with dry powders of the metal salt and the reducing agent as the low concentration of these liquid constituents will result in the overall solids content being at least 75 percent as required.

As noted above, the replenisher composition of the subject invention is in essentially dry form and comprises, using copper plating solutions as an example, one or more ofa copper salt that is the source of cupric ions, paraformaldehyde as a source of formaldehyde, a small amount of chelating agent for cupric ions, stabilizers, brighteners, surfactants and the like. The materials contemplated for use in the subject invention are old, exemplified in the above referenced patents and conventionally used in the formulation of electroless copper deposition solutions.

Typical examples of copper salts for purposes of the present invention include cupric sulphate, cupric chloride, cupric nitrate, cupric hydroxide, cupric tartrate and cupric acetate. Typical examples os suitable complexing agents for the cupric ions include Rochelle salts. the sodium salts (mono-, di-, tri-. and tetrasodium salts) of ethylenediaminetetraacetic acid, nitrilotriacetic acid and its alkali metal salts, tartaric acid, modified ethylenediaminetetraacetic acids such as N- hydroxyethylene-diaminetriacetate, hydroxyalkyl substituted dialkalenetriamines, sodium salicylate and sodium tartrate. Other complexing agents for cupric ions are disclosed in U.S. Pat. Nos. 2,996,408; 3,075,855 and 2,938,805 all incorporated herein by reference.

Materials known in the art as catalytic poisons to the deposition of electroless metals are frequently used in controlled amounts as stabilizers for the plating solutions. The most widely used group of compounds of this nature are the divalent sulphur-containing compounds, many of which are disclosed in U.S. Pat. No. 3,352,540 and exemplified by inorganic sulfides such as sodium sulfide, organic and inorganic thio-compounds such as sodium thiocyanate and organic sulphur compounds such as thiourea. Another class of stabilizers are the water soluble cyanide compounds as defined in U.S. Pat. No. 3,310,430 such as sodium cyanide. A third class of stabilizers is disclosed in U.S. Pat. No. 3,457,089 and includes acetyllinic alcohols and ethers such as 2-butyne-l, 4-diol. Other stabilizers are known in the art.

The relative proportions of the ingredients comprising the dry replenisher mixture may vary within broad limits dependent upon the requirements and use of the electroless copper deposition solutions to which the replenisher is added. In general, the copper salts and paraformaldehyde are added in about the same relative proportions as found in fresh electroless copper depositions solutions, e.g., typically paraformaldehyde in an amount capable of yielding at least 0.5 moles of formaldehyde per mole of copper salt and preferably from 2 to 4 moles per mole of copper. If a chelating agent is used as a constituent of the dry replenisher mixture, its relative proportion in the mixture is substantially less than in the original copper deposition solution as it is not consumed in the plating operaton. Typically, the chelating agent may constitute from about 0.01 to 0.50 moles per mole of copper salt.

The dry replenisher may contain substituents in addition to those noted above. For example, additives may be used to facilitate production such as molding agents, of which glycelol is a good example as it is also a chelat ing agent for the cupric ions. Other additives may include agents to protect against decomposition, sublimation, handling and the like. As noted above, any of the chelating agent, stabilizer, surfactant and brightener, since added in minor proportion, may be in liquid form if blended with the cupric salt, paraformaldehyde or both as the amount of such additive, together with the solid components will result in a replenisher in essentially dry form having a solids content in excess of percent by weight.

In addition to the above ingredients, hydroxide may require replenishment to maintain necessary alkalinity, the most common source of hydroxide being caustic. However, caustic cannot be mixed with the other ingredients of the dry replenisher and must be added separately. If the source of hydroxide is cupric hydroxide, it may be mixed with the other ingredients though caustic might still be required to obtain necessary alkalinity. One method for adding caustic in solid form would be to coat the same with a protective coating which would prevent reaction with formaldehyde which coating should be soluble in the plating solution and not detrimental thereto.

The replenisher should not be added to the electroless solution by simply dropping the same onto the surface of the solution as this would cause spontaneous decomposition and interfere with the deposit on a workpiece. As noted above, it is a discovery of this invention that the replenisher is added to a stabilized bath out of contact with air and in a manner to rapidly disperse powders and to dissolve the same. This is accomplished by adding the replenisher to the replenishment chamber of the apparatus of this invention which apparatus facilitates dispersing the dry replenisher in the solution and dissolving the same as rapidly as possible.

As noted above, the bath to which the replenisher is added should be substantially stable. Upon depletion of a plating bath, there may also be depletion of stabilizer through drag-out or otherwise. Thus, the bath should be stabilized prior to dry replenishment. This may be accomplished in one of several methods. The stabilizer may be blended with the other replenisher ingredients or added first. Alternatively, the initial solution may be loaoded with an excess of stabilizer provided it does not poison the bath or reduce the plating rate to an unacceptable level. Alternatively, the bath may be maintained stable by bubbling air through the same as is known in the prior art. What cannot be done is the addition of the stabilizer to an unstable bath subsequent to replenishment of other ingredients.

The apparatus for adding replenisher is illustrated in the drawings where in FIG. 1, there is illustrated an apparatus comprising a plating section to the right and a replenishment section, greatly enlarged for purposes of illustration, to the left. The plating section consists of plating apparatus 100 of a catalytically inactive material such as polyethylene or polypropylene. This section is divided into plating tank 101 and overflow tank 102. Catalytically active parts are plated in plating tank 101 by racking the same and immersing them in the plating tank (parts and racks not shown). Contact of the catalytically active part with the solution in the plating tank will result in plating.

Plating tank 101 is separated from overflow tank 102 by barrier 103 having overflow lip 104. As the plating solution passes over barrier 103 from plating tank 101 into overflow tank 102, it may pass through filtering means 105 such as a cloth bag. Within overflow tank 102, there may be placed a heat exchanging device 106 to maintain a desired solution temperature. Thus,'in normal use without replenishment, the flow of plating solution is in a closed loop from plating tank 101 over the overflow lip 104 into overflow tank 102, out of overflow tank 102 through outlet 107 having valve 108 in the open position and through pump 109 back into plating tank 101 through a return sparger line 110 and solution sparger 111. During this operation, valves 112 and 113 are in the closed position.

When replenishment is desired, valves 108 and 112 are in the open position and valve 113 is in the closed position. Solution then flows out of overflow tank 102 through outlet 107 and pump 109 into the replenishment apparatus 200 through line 201, valve 112 and inlet 202. Line 201 is sized to allow approximately 75-90 percent of the output from pump 109 to continue recirculating through plating apparatus 100. Inlet 202 discharges into a cyclindrical chamber 203 designed to affect a swirling motion to the solution. The

relationship of inlet 202 to cyclindrical chamber 203 can be seen in more detail in FIG. 2 which is a crosssectional view taken along section line 2-2. From FIG. 2, it can be seen that the inlet 202 discharges into the side of chamber 203 to cause a swirling motion. Within chamber 203 there are contained dry replenisher powders 204 which powders are wetted and passed through disperser plate 205 containing openings 206. The powders 204A wetted with solution pass into a porous container 207, such as a cloth bath made of polpropylene felt, held in chamber 208. The raining action of the solution as it pases through disperser plate 205 causes further dissolution of the powder in porous container 207 at a relatively constant rate. The replenished relatively concentrated solution then passes from chamber 208 through oulet 209, through pipe 210 and into the filter bag which may be of the same material as porous container 207, but with decreased porosity. The filter bag 105 insures that no undissolved constituents enter the solution to form localized areas of high concentration and/or nucleating sites which would result in the aforesaid problems.

The replenisher powders 204 are added to the replenisher apparatus 200 by any convenient means. For example, with reference to the drawing, there is depicted an assembly comprising a hand turn wheel 211 secured with yoke 212. When turning the wheel, cap 213 is raised and the entire assembly moves upwards, and pivots sideways on pivot point 214A or 214B. With the removal of cap 213 there is ready access to upper chamber 203. If it is desired to change or replace porous container 207, this is readily accomplished by removing disperser plate 205 and withdrawing bag connector 216 through upper chamber 203. Then, clamp 220 is removed from bag connector 216 and container 207 readily slips off bag connector 216.

The size of chamber 208 and the length of porous container 207 may vary depending upon the combined volumes of tanks 101 and 102 and the rate of solution I flow through the apparatus. In this respect, it can be seen that container 203 is connected to container 208 mechanically and can be readily separated therefrom. Bag connector 216 is provided with a shoulder which sits on flange 217. To separate container 203 from container 208, screws 218 are removed and flange 217 is separated from flange 219. Pressure is maintained between chambers 203 and 208 with O-ring 221.

Should it be desired to drain chamber 208 after the dry replenisher is dissolved and/or prior to a subsequent dry replenisher addition, this may be accomplished by closing valve 112, removing cap 213 and opening valve 113. This will permit pump 109 to drain chamber 208 and pipe 210 returning solution to tank '101 through sparger line and sparger outlet 111.

Should it be desired to drain tank 101, this can be accomplished by removing cap 115 and permitting the fluid to flow out of the tank. Pump 109 is used to drain solution from tank 102 into tank 101 and out through tank 101 drain 115.

It should be understood that while the replenisher has been referred to as essentially dry powders, it may be in the form of a molded shape, such as that of a tablet or alternatively packaged in a solution soluble plastic bag such as a bag formed from carboxyl methyl cellulose.

EXAMPLE 1 With reference to the apparatus depicted in the drawings, a tank having a capacity of 25 gallons was filled with an electroless copper depositing solution having a formulation as follows:

cupric sulphate pentahydrate 1.5 pounds paraformaldehyde l.5 pounds sodium hydroxide (25% solution by weight) 1 gallon pentahydroxypropyldiethylenetriamine 3.5 pounds water to 20 gallons In addition to the above ingredients, there are added various solution stabilizers and other additives known in the art.

The above solution is maintained at 125F by recirculating the same from the plating chamber to the overflow tank containing a heat exchanging device and filter bag at a rate of 6-12 or more volume turnovers per hour. The solution is used to deposit copper on a catalyzed non-conductive substrate in the manufacture of printed circuit boards with a loading of 1 square foot per gallon of plating solution. After minutes, the solution is depleted in copper, formaldehyde and sodium hydroxide by about 10 percent of the original amount. After 20 minutes, there is a noticeable decrease in plating rate. A powder mixture is prepared comprising about one-fourth pound of dry replenisher constituents blended together. The blend is placed in the replenishment apparatus depicted in the drawing. The solution is then recirculated from the overflow tank through the replenishment apparatus at a rate of about 180 gallons per hour and back into the filter bag of the overflow tak where it is filtered and recirculated into the plating tank. Within about 20 minutes, plating rate returns to normal and powder constituents in the bag are substantially dissolved. At this point, the flow of fluid through the replenishment apparatus may be discontinued.

A further advantage of the dry replenisher apparatus and process of the invention is the ability to maintain the primary components of a plating solution at a predetermined and desired level. As previously described, when the operating parameters of the bath are known, such as constituent concentrations, bath temperature, surface area in relation to bath volume, and plating time, the amount of solution depletion can be predetermined and anticipated Similarly, when this apparatus is used as described, and the solution flow is controlled while maintaining operating temperature, the rate of dissolution of the dry replenisher into the plating solution can be predicted and controlled. Under the conditions previously described, the rate of dissolving, or dissolution of the replenishers, will be approximately 1 percent per minute. As a net result, the use of a dry replenisher can permit a product user to maintain constituent levels at a desirable point within a relatively small tolerance.

The following are examples of suitable replenisher compositions for 1 liter solutions:

Example 2 Cupric sulphate pentahydrate 12.5 grams paraformaldehyde 12.5 grams Example 3 Cupric sul hate pentahydrate 10 grams paraforma dehyde 10 grams -Continued pentahydroxypropyldiethylene tramine 5 grams Example 4 Cupric sulphate pentahydrate 10 grams paraformaldehyde 10 grams ethylenediaminetetraacetic acid 4 grams tetrahydroxypropylethylenediamine 1 gram Example 5 Cupric acetate 10 grams paraformaldehyde 10 grams mixed sodium potassium tartrate 5 grams Each of the above formulations may be molded into tablets measuring approximately 2 inches in diameter by one-fourth inch in thickness if desired.

We claim:

1. An apparatus for replenishing a metal plating solution with replenishers in essentially dry form to thereby avoid volume growth of the plating solution and to avoid the introduction of essentially dry replenisher directly into a tank holding said plating solution, said apparatus comprising a first chamber positioned above a second chamber, said first chamber having inlet means for entry of plating solution and means for supporting said essentially dry replenisher in said chamber, said second chamber having outlet means for exit of plating solution and a porous container placed therein such that the plating solution must pass through said porous container before exiting from said second chamber.

. said supporting means being adapted to permit parti cles of replenisher which have been wetted by said plating solution to pass through said first chamber and to rain down into said porous container in said second chamber, and said first and second chambers being in pressure seal relationship.

2. The apparatus of claim 1 where means are provided in said first chamber for imparting a swirling motion to said plating solution as it pases through said chamber.

3. The apparatus of claim 1 wherein said porous container in said second chamber is of a synthetic plastic felt.

4. The apparatus of claim 3 wherein said synthetic plastic is polypropylene.

5. In combination, a tank for plating metal on a substrate and an apparatus for replenishing a metal plating solution with replenishers in essentially dry form to thereby avoid volume growth of a plating solution and to avoid introduction of replenisher powders directly into said tank holding said plating solution, said apparatus for replenshing said plating solution comprising a first chamber positioned above a second chamber, said first chamber having inlet means for entry of plating solution and means to place replenisher powder in said chamber, said second chamber having outlet means for exit of plating solution and a porous container placed therein such that the plating solution must pass through said porous container before exiting from said second chamber, said first and second chambers being separated by a plate having holes therein providing open communication between said chambers whereby powders of replenisher are placed on said plate which powders are wetted with plating solution passing through said first chamber and said wetted powders are rained into said porous container in said second chamber, said first and second chambers being in pressure seal relationship, said plating tank having inlet and outlet means where the inlet means of the plating tank is connected to the outlet means of the replenisher apparatus and where the outlet means of the plating tank is connected to the inlet means of the replenisher apparatus.

6. The combination of claim where the outlet means of the plating tank has means for dividing a solution passing there through into two streams such that a portion of the solution passes to the inlet means of the replenisher apparatus and the other portion returns to the plating tank through additional inlet means.

7. The combination of claim 6 where the means for dividing the solution is such that a major portion of the solution is returned to the plating tank and a minor portion enters the replenisher apparatus.

8. The combination of claim 6 where the additional inlet means in said plating tank is a solution sparger.

9. The combination of claim 5 where the replenisher apparatus has means provided in said first chamber for imparting a swirling motion to the plating solution as it passes through said first chamber.

10. The combination of claim 5 where the porous container in the second chamber of the replenisher apparatus is of a synthetic plastic material.

11. The combination of claim 10 where said synthetic plastic material is a polypropylene felt.

12. The combination of claim 5 where said plating tank is divided into a plating chamber and an overflow chamber and said inlet and outlet means are connected to said overflow chamber.

13. The combination of claim 12 where said overflow chamber has heat exchange means to maintain a predetermined temperature in said plating tank.

14. The combination of claim 13 where said inlet means discharges into a porous container.

15. The combination of claim 14 where the porous container is of polypropylene felt. 

1. An apparatus for replenishing a metal plating solution with replenishers in essentially dry form to thereby avoid volume growth of the plating solution and to avoid the introduction of essentially dry replenisher directly into a tank holding said plating solution, said apparatus comprising a first chamber positioned above a second chamber, said first chamber having inlet means for entry of plating solution and means for supporting said essentially dry replenisher in said chamber, said second chamber having outlet means for exit of plating solution and a porous container placed therein such that the plating solution must pass through said porous container before exiting from said second chamber, said supporting means being adapted to permit particles of replenisher which have been wetted by said plating solution to pass through said first chamber and to rain down into said porous container in said second chamber, and said first and second chambers being in pressure seal relationship.
 1. An apparatus for replenishing a metal plating solution with replenishers in essentially dry form to thereby avoid volume growth of the plating solution and to avoid the introduction of essentially dry replenisher directly into a tank holding said plating solution, said apparatus comprising a first chamber positioned above a second chamber, said first chamber having inlet means for entry of plating solution and means for supporting said essentially dry replenisher in said chamber, said second chamber having outlet means for exit of plating solution and a porous container placed therein such that the plating solution must pass through said porous container before exiting from said second chamber, said supporting means being adapted to permit particles of replenisher which have been wetted by said plating solution to pass through said first chamber and to rain down into said porous container in said second chamber, and said first and second chambers being in pressure seal relationship.
 2. The apparatus of claim 1 where means are provided in said first chamber for imparting a swirling motion to said plating solution as it pases through said chamber.
 3. The apparatus of claim 1 wherein said porous container in said second chamber is of a synthetic plastic felt.
 4. The apparatus of claim 3 wherein said synthetic plastic is polypropylene.
 5. In combination, a tank for plating metal on a substrate and an apparatus for replenishing a metal plating solution with replenishers in essentially dry form to thereby avoid volume growth of a plating solution and to avoid introduction of replenisher powders directly into said tank holding said plating solution, said apparatus for replenshing said plating solution comprising a first chamber positioned aboVe a second chamber, said first chamber having inlet means for entry of plating solution and means to place replenisher powder in said chamber, said second chamber having outlet means for exit of plating solution and a porous container placed therein such that the plating solution must pass through said porous container before exiting from said second chamber, said first and second chambers being separated by a plate having holes therein providing open communication between said chambers whereby powders of replenisher are placed on said plate which powders are wetted with plating solution passing through said first chamber and said wetted powders are rained into said porous container in said second chamber, said first and second chambers being in pressure seal relationship, said plating tank having inlet and outlet means where the inlet means of the plating tank is connected to the outlet means of the replenisher apparatus and where the outlet means of the plating tank is connected to the inlet means of the replenisher apparatus.
 6. The combination of claim 5 where the outlet means of the plating tank has means for dividing a solution passing there through into two streams such that a portion of the solution passes to the inlet means of the replenisher apparatus and the other portion returns to the plating tank through additional inlet means.
 7. The combination of claim 6 where the means for dividing the solution is such that a major portion of the solution is returned to the plating tank and a minor portion enters the replenisher apparatus.
 8. The combination of claim 6 where the additional inlet means in said plating tank is a solution sparger.
 9. The combination of claim 5 where the replenisher apparatus has means provided in said first chamber for imparting a swirling motion to the plating solution as it passes through said first chamber.
 10. The combination of claim 5 where the porous container in the second chamber of the replenisher apparatus is of a synthetic plastic material.
 11. The combination of claim 10 where said synthetic plastic material is a polypropylene felt.
 12. The combination of claim 5 where said plating tank is divided into a plating chamber and an overflow chamber and said inlet and outlet means are connected to said overflow chamber.
 13. The combination of claim 12 where said overflow chamber has heat exchange means to maintain a predetermined temperature in said plating tank.
 14. The combination of claim 13 where said inlet means discharges into a porous container. 